1. Field of the Invention
The present invention relates to a process for producing a cold-rolled strip or sheet of an austenitic stainless steel by cold-rolling a cast strip having a thickness close to or slightly greater than that of a product strip or sheet, the cast strip being cast by a synchronous type continuous caster in which a cast strip and a casting mold wall are moved without a relative speed difference therebetween.
2. Description of the Related Art
The conventional process for producing a cold-rolled strip or sheet of a stainless steel by using a continuous casting method comprises: casting a melt of a stainless steel into a cast slab having a thickness of 100 mm or more, under a forced oscillation of the casting mold; subjecting the cast slab to a surface treatment or cleaning; heating the slab to a temperature of 1000.degree. C. or higher, followed by a hot-rolling by a hot strip mill comprising rough-rolling stands and finish-rolling stands to form a having a thickness of several millimeters; annealing or not annealing the hot strip; descaling the hot strip; cold-rolling the hot strip followed by a final annealing; and skin-pass-rolling the annealed strip.
The conventional process has a problem in that a long and large line of hot-strip mills is required to hot-roll a cast slab having a thickness of 100 mm or more, and that a great amount of energy is consumed in heating and hot-rolling the cast slab.
To solve these problems, research has been carried out into the development of a process for producing a cast strip having a thickness equivalent or close to that of a hot strip. These processes include a synchronous type continuous casting, in which a cast strip and an inner wall of a casting mold are moved without a relative speed difference therebetween, such as a twin roll process or a twin belt process, as reported in "Tetsu-to-Hagane (Iron and Steel)", 1985, page A197-A256, for example.
Nevertheless, problems still arise when producing a cold-rolled strip or sheet product of a stainless steel by using such a synchronous type continuous casting process, in that a final product strip or sheet has a relatively poor surface appearance due to the reduction or omission of the steps of the process from a cast strip to the product.
The present inventors cast a melt of JIS SUS 304 stainless steel, a most widely used austenitic stainless steel, into a cast strip in the form of a thin strip, by using an inside water-cooled type twin-roll continuous caster, and cold-rolled the cast strip to form a cold-rolled strip or sheet which was then annealed and pickled to obtain a "2B" product strip or sheet (specified by JIS G4305) or bright-annealed to obtain a "BA" product strip or sheet (specified by JIS G4305). A study showed that the surface of a product sheet obtained by using a twin-roll type continuous caster exhibits a characteristic appearance of fine wrinkles or fine crepe-like undulation (hereinafter referred to as "roping") and a nonuniform luster or uneven gloss.